It had submitted a request for appeal in January over the negative federal permitting decision issued in late November, for the project it describes as the second largest undeveloped copper project globally and the world's largest undeveloped gold resource.
The US Army Corps of Engineers accepted the company's request in February and had appointed a review officer to oversee the administrative appeal process.
"While federal guidelines suggest the appeal should conclude within 90 days, the USACE has indicated the complexity of issues and volume of materials associated with Pebble's case means the review will likely take additional time," Northern Dynasty said yesterday.
Its request for appeal is 92 pages long.
In it, the company's US-based subsidiary Pebble Limited Partnership has asserted the USACE's findings of economic detriments were "vague and speculative".
Alaskan governor Mike Dunleavy separately said in January his office would also appeal the USACE's November record of decision, saying it was "flawed" and created a dangerous precedent that would undoubtedly harm Alaska's future.
The proposed project has long faced criticism over environmental and fisheries concerns.
It had been effectively vetoed during the Obama administration but made progress under Donald Trump's presidency.
Northern Dynasty suspected political interference in the November decision, coming weeks after the election of new president Joe Biden, who had publicly voiced opposition to the project.
The company positioned Pebble as "helping to secure America's green future" in a presentation this month, saying it was a potential domestic solution to the US foreign supply chain dependence of critical minerals.
It expected Pebble to produce an average annual 318 million pounds of copper, 362,000 ounces of gold, 1.8Moz of silver, 14 million pounds of molybdenum and 12,000kg of rhenium over 20 years.
However it's been looking for a "lucky number seven" major partner after First Quantum Minerals bowed out of an option agreement in 2018.
Northern Dynasty shares (TSX: NDM) have spanned C38.5c-$3.28 over the past year and closed up 3.3% to 96c, valuing it at $484.4 million (US$388 million).
